Holmes, Watson, and Miss Morstan have been told that the treasure is enormously valuable. Dr. Watson writes:
"Holmes declares that he overheard me caution him against the great danger of taking more than two drops of castor-oil, while I recommended strychnine in large doses as a sedative."
Obviously intended as humor. But strychnine is one of the most horrible poisons of all, because death comes slowly by inability to breathe, and great suffering.

From the Internet:
The onset of symptoms may occur with a
prodromal syndrome that includes: tonic
twitching of the face and neck muscles,
muscular cramps in the legs, preceded by
restlessness, apprehension, and heightened
acuity of perception (hearing, vision,
feeling) and hyperreflexia. However, the
initial symptoms may be only generalized
violent convulsions. These convulsions can
begin suddenly after any minor sensory
stimulus and last from 50 seconds to 2
minutes. Initially, the convulsions are
clonic but are soon followed by tonic
contractions similar to convulsions due to
tetanus.

The patient remains conscious and has intense
pain. After the convulsions, all the muscles
relax and sometimes the patient falls asleep
from exhaustion. Hyperexcitability recurs
suddenly after 10 to 15 minutes.


Repeated convulsions (1 to 10) are common
before recovery or death. In severe
untreated poisoning, each convulsion lasts
longer than the previous one, and the
intervals between them are shorter.